Patients who are successfully resuscitated from prehospital cardiac arrest constitute a unique population for study because of their susceptibility to both acute and long-term recurrent, potentially lethal ventricular arrhythmias. Because of this electrophysiologic instability, we are studying electrophysiologic and clinical characteristics of these patients. The protocol is addressed to studies of 3 major areas of investigation: (1) the acute electro-physiologic characteristics of survivors of the initial event during the first 72 hours after cardiac arrest; (2) abnormalities of lactate metabolism which underlie a dissociation in oxyhemoglobin affinity and red blood cell 2,3 diphosphoglycerate levels previously identified in these patients; and (3) the clinical electrophysiologic characteristics of late chronic ventricular arrhythmias and the effects of antiarrhythmic drugs on chronic arrhythmias. The acute electrophysiologic studies include intracardiac recordings and electrophysiologic characteristics identified by body surface recordings. The long-term electrophysiologic studies include identification of rhythm disturbances on 24-hour Holter monitor recordings carried out monthly and correlated with fluctuation in blood levels of antiarrhythmic drugs. Similar short-term (8 hours) studies are being carried out to determine the relationship between diurnal variations in frequency of arrhythmias and corresponding fluctuations in blood levels of antiarrhythmic agents.